Prostate cancer, which is now the most commonly diagnosed cancer in United States males, displays a wide range of biological activity, such that its classification into diagnostic and prognostic categories is difficult. Recently, neuroendocrine (NE) differentiation in prostatic carcinomas has been associated with tumor progression, resistance to androgen-ablation therapy and poor prognosis. Hormonal substances such as serotonin (5-HT), bombesin/gastrin-releasing peptide (GRP), calcitonin, parathyroid hormone and other neuropeptides found in prostate tumors with NE differentiation play an important role in determining tumor cell growth and metastatic behavior. Clinically, these NE factors could be useful markers of tumor progression and also serve as therapeutic targets. This study will focus on the involvement of the monoamine 5-HT in the growth of androgen-independent prostate tumor cells. Antiproliferative effects of subtype-selective 5-HT receptor antagonists, 5-HT synthesis- and uptake-inhibitors on three human prostate tumor cell lines PC-3, DU-145 and LNCaP will be examined in vitro and in vivo, in nude mice. Growth- inhibitory effects of combinations of 5-HT, bombesin/GRP and vasoactive intestinal polypeptide (VIP) antagonists and also combinations with somatostatin analogs and interferon alpha will be investigated in order to provide a basis for alternative therapeutic strategies. To characterize the 5-HT receptor, radioligand binding assays and cAMP assays will be performed. The association of 5-HT and bombesin/GRP expression with metastasis will be studied. Development of bone metastases is a seminal event in the progression of prostatic carcinoma. Association of expression of 5-HT and bombesin/GRP with urokinase, a proteolytic enzyme strongly associated with the metastatic phenotype, will be examined in bone marrow biopsies. Modulation of urokinase enzyme activity and mRNA levels in the three cell lines by NE factors will be also be tested. Urinary levels of the 5-HT metabolite 5-HIAA and of bombesin-like immunoreactivity will be measured in prostate cancer patients at various stages of the disease. These measurements could be useful for the selection of patients likely to respond to antagonists to NE factors and for monitoring therapy. Prostate tumor cells with NE markers chromogranin A, neuron-specific enolase and 5-HT, but do not express pSA. Interestingly, these two cell lines secrete a trypsin- and acid-sensitive, but heat-stable factor (MW more 30 kD) which inhibits the synthesis and secretion of PSA by LNCaP cells. This PSA-suppression factor present in pC-3 and DU-145 conditioned medium will be purified and characterized.